This invention relates to a filter plate for a plate type filter press, which is particularly adapted for use in the dewatering of sewage sludges.
As is well known, any number of such filter plates can be slidably attached side by side to the usual horizontal guide means of the filter press frame. The filter plates are provided with suitable filtrate discharge ports or the like and are covered on both sides by filter cloth which is disposed at the bottom of the cake chambers formed by recesses in the filter plates. When the filter press is ready for operation, the filter plates are moved from their spaced apart locations and pushed closely together and hydraulically pressed against each other. The suspension to be dewatered is then introduced into the filter chambers. The solids remain on the filter media forming filter cakes thereon, whereas the filtrate passes through the cloth and then through the usual discharge ports in the plates whereupon it will be disposed of as desired.
In view of the large size of conventional filter plates used at the present, it is necessary to support the filter plates on each other by at least one staying means in addition to the filter plate supporting frame, with such staying means being provided inside the cake chamber. This is very desirable in view of possible pressure difference building up on either side of a filter plate during introduction of the suspension which results in high bending stresses.
In conventional apparatus cams have been employed to overcome this disadvantage with such cams extending from the chamber essentially out of the level of the plate frame. However, problems are encountered with such cams due to the fact that the filter cloth covers the entire surface of the plate and is secured at its edges.
One attempt to solve this problem was to place the filter cloth over the cam. However, it was found that the cloth was under too much tension at the junction of the cam and filter chamber so as to become unusable after a short period of use.
Later, by way of improvement, such cams were not formed integrally with the filter plate but were fastened thereto by screw means with the filter cloth being inserted and fastened between the plate and the cam. But even this attempt to solve the problem had a serious drawback due to the construction of the plate. The surface against which the cam abuts could not be manufactured and finished with the frame in a single operation. Also, as the filter cloth is later interposed between two metal surfaces its final thickness under compression, cannot be predetermined accurately. Accordingly, considerable differences in height resulted between the outer surface of the cam and the outer surface of the filter cloth near the plate frame with such differences interfering with the support of the plates upon one another.
In addition to these attempts to solve the above mentioned problem the filter cloth has been provided with a cutout for the cam, with the edge of this cutout being sealingly retained by means of a clamping ring. Such a conventional design is shown in the German design patent No. 1,967,244 wherein an O-ring used for this purpose is fitted in a groove at the bottom of the cam. But even this attempt was not satisfactory from a constuctional point of view. First, it was difficult to cut the groove for the O-ring and second, it was still not possible to finish the surface of the cam and the plate frame in a single operation. Since the filter cloth is disposed on the filter frame and not on the cam, the resulting difference in height makes it difficult to support the plates properly.